The invention pertains to ultrasonic sensors and more specifically to ultrasonic sensors having signal processing subassemblies that may be remotely powered.
An ultrasonic sensor, which typically utilizes a transducer that produces an electrical output in response to received ultrasonic energy, is used to locate and measure leaks or defects in pipes and the like as well as to detect excess friction within mechanical devices. The transducer output is coupled to a signal processing subassembly that derives a measurement signal proportional to the transducer output. The measured ultrasonic energy is generally in the range of 20-100 KHz, which is too high in frequency to be heard by a human being. Thus, the signal processing subassembly is sometimes adapted to frequency shift the detected signal into the 0-20 Khz audio range. In some cases, the subassembly includes facilities for deriving monitoring a DC signal proportional to such audio signal.
Because of the low levels of the ultrasonic signals detected by a sensor of this type, it has been necessary to augment the detected signal using a high-gain preamplifier before it can be further processed. In order to prevent such amplified signal from saturating the heterodyne and measurement circuitry, it has been common to attenuate the preamplified signal. Unfortunately, with such arrangements the maximum device sensitivityxe2x80x94i.e., the dynamic range of ultrasonic signal inputs that can be accurately processed by the sensorxe2x80x94has been limited typically to the range of 30-40 DB.
Such limitations on dynamic range are minimized with an ultrasonic sensor in accordance with the invention. In an illustrative embodiment, the preamplified output of an ultrasonic transducer is chopped by the output of a free-running oscillator which generates a selectable frequency differing from the frequency of the transducer output by an amount within the audio frequency range. The output of the chopper includes a signal having a frequency equal to the difference of the frequencies of the transducer and the oscillator.
The frequency components of such difference signal are selectively amplified to concentrate the spectral energy of the difference signal in a lower portion of the audio range. A low pass filter extracts, from the so-concentrated difference signal, a modified audio signal whose frequency content is at the lower end, illustratively 0-6 KHz, of the spectrum of the concentrated difference signal.
Preferably, the output of the low pass filter is coupled to an AC to DC converter which provides a DC voltage that is proportional to the ultrasonic energy level detected by the transducer. As an additional feature of the invention, the AC to DC converter utilizes a compression network that prevents the converter output from saturating over the enhanced dynamic range of the input signal.
The improved sensitivity provided by the arrangement of the invention allows the sensor to be employed effectively with an external power supply. Illustratively, the sensor is connected to a 4-20 mA current loop which may serve both as a vehicle for powering of the signal processing subassembly of the sensor as well as for carrying an output current proportional to the DC voltage generated by the sensor. Such current may be generated by a suitable voltage to current converter in the signal processing subassembly of the sensor.